It's not because the idea is horrible, it's because it'd be a pain in the arse to do it. This has been suggested before (search!
), but it's just too hard...
First of all, you'll need a decent dictionary which can be used in a script. Therefore, you need a huge library of words and find a way to easily generate a list of suggestions.
Second, you'll need to detect on what word the cursor is pointed. This is almost impossible, because you can change your font and thus affect where the word is placed. However, Plus! did something similar with the Quick Emoticon, but therefore you'll have to ask Patchou how that has been done because I have no clue how.
I doubt this is going to be made any soon...
/ 
